Staff at the creche in Neuilly-sur-Marne, in the eastern Paris suburbs, raised the alarm after a 16-month-old child became dizzy and sleepy after snack-time on June 5.
The child was hospitalised for three days and tests found traces of benzodiazepine — a sedative and muscle relaxant — in their blood, hospital management told AFP.
Two weeks later a nine-month-old boy at the creche — which looks after the children of hospital staff — showed the same symptoms and was also placed
under observation, prompting prosecutors to open an investigation.
The creche has been under heightened security, with parents forbidden from accessing it and bottles prepared for the babies only in the presence of two people.
The suspect, who had worked on and off for the creche for two years as a cleaner, was detained in mid-July. She was suspended from her job on Monday, the hospital said.
A source close to the case said investigators had found a prescription at her home for medication containing formulas corresponding to the drug found in tests on the first child.